Non-volatile memory devices are used in certain applications where data must be retained when power is disconnected. Applications generally include memory cards, consumer electronics (e.g., digital camera memory), automotive (e.g., electronic odometers), and industrial applications (e.g., electronic valve parameter storage). Non-volatile memories may use phase-change memory materials, e.g., materials that can be programmed between a generally amorphous and a generally crystalline state, for electronic memory applications. This type of memory generally includes an array of memory elements, wherein each memory element defines a discrete memory location. Each memory element may include a volume of phase-change material and at least one electrode.
One type of known memory element utilizes a phase-change material that may be programmed between a generally amorphous state and generally crystalline local order. In addition, the phase-change material may be programmed between different detectable states of local order across the entire spectrum between a completely amorphous state and a completely crystalline state. These different structured states have different values of resistivity, and therefore each state can be determined by electrical sensing. Typical materials suitable for such application include those utilizing various chalcogenide materials. Unlike certain known devices, these electrical memory devices typically do not use field effect transistor devices as the memory storage element, but may comprise, in the electrical context, a monolithic body of thin film chalcogenide material. As a result, very little chip real estate is required to store a bit of information, thereby providing for inherently high density memory chips.
One characteristic common to both solid state and phase-change memory devices is significant power consumption, particularly when setting or reprogramming memory elements. Generally, the electrical energy required to produce a detectable change in resistance in these materials is approximately 100 picojoules. This amount of energy must be delivered to each of the memory elements in the solid state matrix of rows and columns of memory cells. Such high energy requirements translate into high current carrying requirements for address lines and cell isolation/access devices that are associated with each discrete memory element. Electrodes (also referred to as electrical contacts) used to supply heat to the phase-change memory material can also have a significant effect on these energy requirements. Generally, higher resistivity electrodes will generate more heat and reduce energy consumption.
Another characteristic common to both solid state and phase-change memory devices is that both have a limited reprogrammable cycle life, i.e., the number of times the device can be programmed from an amorphous state to a crystalline state, and vice versa. Further, over time the phase-change memory material can fail to reliably reprogram between an amorphous and crystalline state. Instability in the resistivity of the electrical contacts or electrodes used to supply heat to the phase-change memory material can exacerbate this reliability problem. It would be advantageous to increase the programmable cycle life of a phase-change memory material and to improve the reliability and stability of the memory devices incorporating them.
A disadvantage of known electrodes used with phase-change memory devices is that the electrodes tend to be chemically reactive with their associated phase-change material. This reactivity degrades the performance of the memory device and results in delamination of phase-change material or in a chemical compositional change to the phase-change material, which can adversely affect the device memory characteristics.
In addition, certain known electrodes used in memory devices have surfaces that are textured, uneven, or rough. Relatively thin layers (on the order of Angstroms) of insulators, electrodes, and phase-change memory materials are typically used in memory devices. Thus, an uneven electrode surface can cause the electrode to protrude through a portion of the phase-change chalcogenide material, resulting in an adverse impact on its memory characteristics.
Thus a need has arisen for an electrode and memory device that addresses one or more of the foregoing disadvantages.